Low-Rate FEC for $M$-ary Orthogonal Modulation

The use of standard binary turbo codes has previously been shown to work well with noncoherent $M$-ary orthogonal modulation, particularly when used with iterative demodulation and decoding at the receiver. For channels with very low signal-to-noise ratios, the code rate may need to be reduced below those provided by typical turbo codes. Rather than designing additional lower rate turbo codes for these cases, repetition can be used. One possibility to incorporate repetition is to repeat individual $M$-ary orthogonal channel symbols, which can be noncoherently combined at the receiver. An alternative approach is to repeat and interleave code bits prior to mapping them to channel symbols. In this paper, this bit repetition approach, combined with a receiver utilizing an iterative demodulation and decoding technique in which the demodulator itself is also iterative, is shown to outperform symbol repetition.